The invention relates to a process for producing electrodes for solar cells, the electrode being configured as an electrically conductive layer on a substrate for solar cells.
Solar cells comprise generally a semiconductor substrate with a number of p- and n-doped regions which generate a potential difference and a voltage with one another when they are exposed to sunlight. In order to be able to draw off the voltage, electrodes are applied to the surfaces of the semiconductor substrate. At present, the electrodes are generally applied in a screenprinting process. The production of electrodes in a screenprinting process is described, for example, in EP-A 1 911 584, US-A 2007/0187652 or U.S. Pat. No. 4,375,007.
As an alternative, for example, WO 2008/021782 discloses first applying a metal layer to the semiconductor material, applying a covering resist by an inkjet printing process, which covers the regions which are to form the structure of the electrodes, and then removing the uncovered regions of the metal layer by an etching process. Subsequently, the covering resist is removed again.
The introduction of contact orifices into a passivation layer on a semiconductor substrate, for example by means of laser-based systems, is described in EP-A 1 833 099. After the introduction of the contact orifices, a direct-writing metalization process introduces a metal into the contact orifices. Examples of direct-writing metalization processes mentioned are inkjet processes or extrusion processes. Finally, a high-conductivity material is applied to the contact material deposited beforehand and between the contacting orifices.
DE-A 10 2006 033 887 discloses applying an electrically conductive layer to a substrate, by transferring a transfer layer comprising an electrically conductive polymer from a transfer film to the substrate.
One disadvantage of the printing and embossing processes known from the prior art is that printing resolution is limited especially in the case of screenprinting, and conductor tracks with a width of less than 120 μm cannot be printed. Efficient power generation in solar cells, however, requires a maximum usable surface area, which is why it is desirable also to print conductor track structures with smaller dimensions.
A further disadvantage of the printing and embossing processes is that they do not proceed contactlessly and the substrate can fracture owing to the pressure applied by the contact, for example with screen and blade in the course of screenprinting. In contactless processes, no pressure is exerted on the substrate, and so the risk of fracture of the substrate is significantly reduced. The contactless processes known from the prior art are generally etching processes, which have the disadvantage that acids and alkalis have to be used for the etching and subsequent removal of the covering resist. In addition, several complicated process steps are required.